A transmission path of a mobile communication system usually comprises an amplifier or multiple amplifier stages which can be adjusted regarding their output power. To this end such amplifiers which can also be called variable gain amplifiers, VGA, comprise a control input for adjusting a variable gain factor of the amplifier.
In many cases a fine tuning of the output power can be required within a wide output power range, especially in systems for modern mobile communication standards as universal mobile telecommunication standard, UMTS. For this purpose amplifiers with a gain factor which can be adjusted in an analogue way in response to an analogue control signal are widely used.
Although a small absolute change of the amplifier gain can be achieved using conventional amplifier arrangements, an effective gain factor of the amplifier arrangement can be dependent on the respective control signal in a non-linear way. Accordingly, a constant ratio between adjusted gain factor and provided control signal which corresponds to a constant slope of the amplifier arrangement is hard to achieve. However, a constant slope can be an important requirement for an exact power control of the amplifier arrangement. Furthermore, it is desirable that the slope of the amplifier is independent of the temperature or process variations.
In many cases amplifier arrangements comprising multiple amplifier stages are used in the transmission path to achieve the required wide output power range. Thereby it can be desirable to dimension the amplifier stages in such a way that a resulting gain of the multi-stage amplifier or a gain characteristic respectively is linearly dependent on a control signal. Deviations of single amplifier stages because of, for example, process or temperature variations can influence the gain characteristic. This again can result in the effect that a desired linear dependence can not or not any longer be achieved.
In conventional amplifier arrangements this issue can for example be addressed with inductive feedback or degeneration. Another possibility to reduce a variation of the gain characteristic is to provide a control arrangement that controls an amplifier or an amplifier stage to maintain a desired current at a desired operating point of the amplifier. Nevertheless deviations of the slope of the gain characteristic can occur using these measures as the gain characteristic is dependent on other factors, too.